1. No, there's no concern about the fingerprint scan. It uses a passcode as a backup, so you should be very secure there.

2. If you need Bluetooth (for a headset or what have you) another great pair of power-saving tips is to turn off the parallax feature (under "Accessibility -> Reduce Motion") and turn off the "app background refreshes" under General -> Background App Refresh for any apps that don't need it (that would be most of them). Here's a few more tips for saving power:

4. I'm sure a third-party "theme" utility will come along in due course, but more likely I'd say Apple will try to further improve battery by gradually turning off some of the more egregious white-out conditions in iOS 7 in future updates.

5. Because it is now so easy to turn off BT and Wi-Fi when they are not needed, get into the habit of doing so when you're going to be away from signal/need for those for a while (like when hiking in a forest or some such).